The present invention relates to a process for separating a mixture of an organic fluid and water, and, more particularly, to a process that involves distillation of the mixture, H.sub.2 O-selective pervaporation and application of reverse osmosis to the distillate.
In Germany alone, several tens of thousands of tons of spent glycol solutions having a concentration of approximately 20% are generated each year by automobile radiators. In the future, it will no longer be permitted that the spent contents of the radiators are directed to the sewage treatment plants but they must be treated as special refuse. A considerable demand therefore exists for a cost-effective possibility of a disposal and recycling of the substances.
A pure distillation method for the separation of the glycol has several disadvantages. For example, because of various components, there is the danger that nitrosamines may be formed at temperatures of above 80.degree. C. This can only be avoided by a high-expenditure complete removal of nitrite and nitrate. For achieving a high purity of the distillate, a high recycle ratio is required which, however, can be achieved only with a high consumption of energy. Moreover, the bottom product formed during the distillation has to be removed separately.
From an economical point of view, regeneration exclusively according to the known pervaporation method is not suitable because the membrane surface required would be very large and expensive.
It is an object of the present invention, therefore, to provide an economical method for the separation of a mixture of water and an organic fluid, particularly glycol, in which case the separated water is obtained with such high purity that reuse or an unlimited introduction is possible.
This object has been achieved by a method in accordance with the present invention in which occurs distilling the mixture; performing an H.sub.2 O-selective pervaporation to bottom product obtained from the distillation step to obtain a residue and permeate; applying a reverse osmosis to at least the distillate to obtain a residue and permeate; and feeding the residue obtained from the reverse osmosis for performing thereon the distillation step, such that segregated organic fluid is present as residue from the H.sub.2 O-selective pervaporation, and segregated water is present as the permeate from the H.sub.2 O-selective pervaporation step as well as the permeate from the reverse osmosis step.
The method according to the present invention, which, as stated above, combines the processes of distillation, with one of pervaporation and reverse osmosis, is, of course, to be clearly understood that the process of the present invention is not limited to the separation of glycol from water. It can be used in general for the separation of fluid mixtures consisting of water and an organic fluid, such as carboxylic acids, including, for example, ethanoic acid, propionic acid; aromatic amines, including, for example, aniline; phenol; and glycerin.
It is a prerequisite for the utilization of the method according to the present invention that such organic fluids are used which can be separated from water by each of the three individual processes of distillation, selective H.sub.2 O-pervaporation and reverse osmosis. The parameters to be determined for the feasibility of a distillation process is the temperature of ebullition (boiling) of the two substances to be separated. In contrast, for the feasibility of a reverse osmosis process, the molar mass is of great importance.
The method according to the present invention can be used for such organic fluids whose temperatures of ebullition (or boiling) in the case of the respective pressure are higher than that of water, and whose molar mass is above 40 g/mol. The state-of-the-art of suitable membranes for carrying out an H.sub.2 O-selective pervaporation has nowadays reached such a high level of sophistication that a suitable membrane is available for any organic fluid.
The method of the present invention permits the organic fluid, which is preferably present in a concentration range of from about 5 to 70%, to be enriched to a concentration of at least 90%, preferably equal to or greater than (.gtoreq.) 95%. At the same time, the segregated water is present with a purity of at least 99.9% and is therefore reusable to an unlimited extent.
According to the present invention, a distillation of the mixture takes place first. Preferable parameters of the process are in this example a temperature of 70.degree. C.-160.degree. C. and a pressure of 100 mbar-1,000 mbar. The distillation may be carried out at a normal pressure as well as at a reduced pressure (i.e. vacuum distillation).
The next step is the concentration of the organic fluid by the application of a pervaporation with the use of water-selective membranes to the bottom product obtained from the distillation. In this case, the preferred parameters of the process are a temperature of 60.degree. C.-95.degree. C., and a permeate-side pressure of 20-150 mbar.
The treatment of the water obtained from the distillate takes place by reverse osmosis. The obtained residue is returned into the feed of the distillation. Preferred parameters of the process in the case of the reverse osmosis are a temperature of 15.degree. C.-35.degree. C. and a pressure of 20-70 bar.
The separated organic fluid is present in the permeate obtained from the pervaporation with a concentration of at least 90%, preferably .gtoreq.95%. The separated water is present as a permeate obtained from the reverse osmosis and as a permeate obtained from the pervaporation. Advantageously, the permeate from the pervaporation can be introduced into the feed of the reverse osmosis in order to increase the purity of the water contained therein. The separated water is further treated by an activated carbon filter.
The method according to the present invention is particularly advantageous from a technical point of view (i.e., the products are a high-quality concentrate as well as a reusable high-purity water) as well as from an economical point of view (low energy requirement, low investment). In particular, the necessary structure for carrying out the method can be adapted to the respectively existing economical/technical conditions and can be optimized. Should, in the future, for example, the pervaporation prove to be particularly cost-effective (e.g., better, less expensive membranes than previously), it will be possible to expand the pervaporation within the overall method and to reduce the expenditures for the distillation process. Likewise, it is possible, should the reverse osmosis develop into a much more effective and more economical process, to increase the expenditures for the reverse osmosis and to reduce the expenditures in the distillation and the pervaporation steps. That is, the demands on the distillate and on the permeate can be reduced.